Most airbag modules in use today are large, heavy, expensive, and inefficient. As a result, airbags are now primarily only used for protecting the passenger and driver in a frontal impact, although at least three automobile manufacturers currently offer a small airbag providing limited protection in side impacts. The main advantage of airbags over other energy absorbing structures is that they utilize the space between the occupant and vehicle interior surfaces to absorb the kinetic energy of the occupant during a crash, cushioning the impending impact of the occupant with the vehicle interior surfaces. Airbags have been so successful in frontal impacts that it is only a matter of time before they are effectively used for side impact protection, protection for rear seat occupants and in place of current knee bolsters. Substantial improvements, however, must be made in airbags before they assume many of these additional tasks
A good place to start describing the problems with current airbags is with a calculation of the amount of energy used in a typical airbag inflator and how much energy is required to inflate an airbag. By one analysis, the chemical propellant in a typical driver's side inflator contains approximately 50,000 foot pounds (68,000 joules) of energy. A calculation made to determine the energy required to inflate a driver's side airbag yields an estimate of about 500 foot pounds (680 joules). A comparison of these numbers shows that approximately 99% of the energy in a chemical propellant is lost, that is, generated but not needed for inflation of the airbag. One reason for this is that there is a mismatch between the output of a burning propellant and the inflation requirements of an airbag. In engineering this is known as an impedance mismatch. Stated simply, propellants naturally produce gases having high temperatures and high pressures and low gas flow rates. Airbags, on the other hand, need gases with low temperatures and low pressures and high gas flow rates.
In view of this impedance mismatch, inflators are, in theory at least, many times larger then they would have to be if the energy of the propellant contained within the inflator were efficiently utilized. Some attempts to partially solve this problem have resulted in a so-called "hybrid" inflator where a stored pressurized gas is heated by a propellant to inflate the airbag. Such systems are considerably more energy efficient, however, they also require a container of high pressure gas and means for monitoring the pressure in that container. Other systems have attempted to use aspiration techniques, but because of the geometry constraints of current car inflator designs and mounting locations, and for other reasons, currently used aspiration systems are only able to draw up to about 30% of the gas needed to inflate an airbag from the passenger compartment. Theoretical studies have shown that as much as 90% or more of the gas could be obtained in this manner.
Furthermore, since inflators are large and inefficient, severe restrictions have been placed on the type of propellants that can be used since the combustion products of the propellant must be breathable by automobile occupants. It is of little value to save an occupant from death in an automobile accident only to suffocate him from an excessive amount of carbon dioxide in the air within the passenger compartment after the accident. If inflators operated more efficiently, then alternate, more efficient but slightly toxic propellants could be used. Also, current inflators are made from propellants, namely sodium azide, which are not totally consumed. Only about 40% of the mass of sodium azide propellants currently being used, for example, enters the airbag as gas. This residual mass is very hot and requires the inflator to be mounted away from combustible materials further adding to the mass and size of the airbag system and restricts the materials that can be used for the inflator.
It is a persistent problem in the art that many people are being seriously injured or even killed today by the airbag itself. This generally happens when an occupant is out-of-position and against an airbag module when the airbag deploys. In order to open the module cover, sometimes called the deployment door, substantial pressure must first build up in the airbag before enough force is generated to burst open the cover. This pressure is even greater if the occupant is in a position that prevents the door from opening. As a result, work is underway to substantially reduce the amount of energy required to open the deployment doors and devices have been developed which pop off the deployment door or else cut the deployment door material using pyrotechnics, for example.
One reason that this is such a significant problem is that the airbag module itself is quite large and, in particular, the airbags are made out of thick, heavy material and packaged in a poor, folded geometry. The airbag, for example, which protects the passenger is housed in a module which is typically about one third as long as the deployed airbag. All of this heavy airbag material must be rolled and folded inside this comparatively small module, thus requiring substantial energy to unfold during deployment. This situation could be substantially improved if the airbag module were to have an alternate geometry and if the airbag material were substantially lighter and thinner and, therefore, less massive and folded mainly parallel to the inflator. Even the time to deploy the airbag is substantially affected by the mass of the airbag material and the need to unfold an airbag with a complicated folding pattern. Parallel folding, as used herein, means that the airbag material is folded with the fold lines substantially parallel to the axis of the inflator without being folded over lengthwise as is now done with conventional airbag folding patterns.
Devices are under development that will monitor the position of the occupant and prevent the airbag from deploying if the occupant is dangerously close to the module where lie or she can be seriously injured by the deployment. Some systems will also prevent deployment if the seat in connection with which the airbag operates is unoccupied. An alternate approach is to move the deployment doors to a location away from normal occupant positions. One such location is the ceiling of the vehicle. One problem with ceiling mounted airbags is that the distance required for the airbag to travel, in some cases, is longer and therefore a larger airbag is needed with greater deployment time. With the use of light airbag materials, such as thin plastic film, as disclosed in the above referenced patent applications Ser. Nos. 08/247,763 and 08/539,676, and the use of more efficient inflators, both of these problems can be solved especially for the front and rear seat passengers. The driver poses a different problem since it would be difficult to position a ceiling mounted airbag module where the airbag would always be projected properly between the occupant and the steering wheel.
This problem for the driver's airbag system is not the concept of mounting the airbag on the ceiling, but the design of the steering wheel and steering column. These designs come from the time when the only way of steering an automobile was through mechanical linkages. The majority of vehicles manufactured today have power assisted steering systems and, in fact, most drivers would have difficulty steering a car today if the power steering failed. If servo power steering were used, the need for a mechanical linkage between a steering wheel, or other such device, and the power steering system would no longer be necessary. Servo power steering for the purposes here will mean those cases where the linkage between the manually operated steering device, which regardless of what that device is, will herein be called a steering wheel, is done with a servo system either electrically or hydraulically and the system does not have an operative mechanical connection between the steering wheel and the steering mechanism which moves the wheels.
The problem of educating the general population, which has become secure in the feeling of a steering wheel and steering column, might be insurmountable if it were not for the substantial safety advantage resulting from substituting servo power steering for conventional steering systems and using a non-steering wheel mounted airbag module for the driver.
The steering wheel and steering column are among the most dangerous parts of the vehicle to the occupant. Small people, for example, who are wearing seatbelts can still be seriously injured or killed in accidents as their faces slam into the steering wheel hubs. The problem of properly positioning an airbag, when the comfort and convenience features of telescoping and tilting steering columns are considered, results in substantial safety compromises. Deployment induced injuries which result when a small person is close to the steering wheel when the airbag deploys have already caused several deaths and numerous serious injuries. Future vehicles, therefore, for safety reasons should be constructed without the massive steering wheel and steering column and substitute therefor a servo steering assembly. With this modification, a ceiling mounted airbag module, such as discussed herein, becomes feasible for the driver as well as the other seating positions in the vehicle.
The front seat of the vehicle today has an airbag for the passenger and another for the driver. In some accidents, an occupant, and particularly a center seated occupant, can pass between the two airbags and not receive the full protection from either one. If a ceiling mounted airbag system were used, a single airbag could be deployed to cover the entire front seat greatly simplifying the airbag system design.
One method of partially solving many of these problems is to use an efficient aspirated airbag system. There have been numerous patents granted on designs for airbag systems using aspirated inflators. In these patents as well as in the discussion herein the term "pumping ratio" is used. The pumping ratio as used in the art is defined as the ratio of the mass of gas aspirated from the environment, either from inside or outside of the vehicle, to the mass of gas generated by burning the propellant. A brief description of several pertinent patents, all of which are included herein by reference, follows:
U.S. Pat. No. 2,052,869 to Coanda illustrates the manner in which a fluid jet is caused to change direction, although no mention is made of its use in airbags. This principle, the "Coanda effect", is used in some implementations of the instant invention as well as in U.S. Pat. No. 3,909,037 to Stewart discussed below. It's primary contribution is that when used in inflator designs, it permits a reduction in the length of the nozzle required to efficiently aspirate air into the airbag. No disclosure is made of a pumping ratio in this system and in fact it is not an object of Coanda to aspirate fluid.
U.S. Pat. No. 3,204,862 to Hadeler also predates the invention of vehicular airbags but is nonetheless a good example of the use of aspiration to inflate an inflatable structure. In this device, an inflating gas is injected into an annular converging-diverging nozzle and some space efficiency is obtained by locating the nozzle so that the flow is parallel to the wall of the inflatable structure. No mention is made of a pumping ratio of this device and furthermore, this device is circular.
U.S. Pat. No. 3,632,133 to Hass provides a good example of a nozzle in a circular module with a high pumping ratio in an early construction of an airbag. Although analysis indicates that pumping ratios of 4:1 or 5:1 would be difficult to achieve with this design as illustrated, nevertheless, this reference illustrates the size and rough shape of an aspirating system which is required to obtain high pumping ratios using the prior art designs.
U.S. Pat. No. 3,909,037 to Stewart provides a good example of the application of the Coanda effect to airbag aspirating inflators. Stewart, nevertheless, still discards most of the energy in the propellant which is absorbed as heat in the inflator mechanism. Most propellants considered for airbag applications burn at pressures in excess of about 1000 psig. Stewart discloses that the maximum efficiency corresponding to a 5:1 pumping ratio occurs at inflator gas pressures of about 5 to about 45 psig. In order to reduce the pressure, Stewart utilizes a complicated filtering system similar to that used in conventional inflators. Stewart requires the use of valves to close off the aspiration ports when the system is not aspirating. Through the use of the Coanda effect, Stewart alludes to a substantial reduction in the size of the aspiration system, compared to Hass for example. Also, Stewart shows only a simple converging nozzle through which the burning propellant is passed.
U.S. Pat. No. 4,833,996 to Hayashi et al. describes a gas generating apparatus for inflating an airbag which is circular and allegedly provides an instantaneous pumping ratio of up to 7:1 although analysis shows that this is unlikely in the illustrated geometry. The average pumping ratio is specified to be up to 4:1. This invention is designed for the driver side of the vehicle where unrestricted access to the aspirating port might be difficult to achieve when mounted on a steering wheel. The propellant of choice in Hayashi et al. is sodium azide which requires extensive filtering to remove particulates. No attempt has been made in this design to optimize the nozzle geometry to make use of a converging-diverging nozzle design, for example. Also, the inflator has a roughly conventional driver side shape. It is also interesting to note that no mention is made of valves to close off or restrict flow through the aspiration port during deflation. Since most aspiration designs having even substantially smaller pumping ratios provide for such valves, the elimination of these valves would be a significant advance in the art. Analysis shows, however, that the opening needed for the claimed aspiration ratios would in general be far too large for it also to be used for exhausting the airbag during a crash. Since this is not discussed, it should be assumed that valves are required but not illustrated in the figures.
U.S. Pat. No. 4,877,264 to Cuevas describes an aspirating/venting airbag module assembly which includes a circular gas generator and contemplates the use of conventional sodium azide propellants or equivalent. The aspiration or pumping ratio of this inflator is approximately 0.2:1, substantially below that of Hayashi et al., but more in line with aspiration systems in common use today. This design also does not require use of aspiration valves which is more reasonable for this case, but still unlikely, since the aspiration port area is much smaller. Again, no attempt has been made to optimize the nozzle design as is evident by the short nozzle length and the low pumping ratio.
U.S. Pat. No. 4,909,549 to Poole et al. describes a process for inflating an airbag with an aspiration system but does not discuss the aspiration design or mechanism and merely asserts that a ratio as high as 4:1 is possible but assumes that 2.5:1 is available. This patent is significant in that it discloses the idea that if such high pumping ratios are obtainable (i.e., 2.5:1 compared with 0.2:1 for inflators in use), then certain propellants, which would otherwise be unacceptable due to their production of toxic chemicals, can be used. For example, the patent discloses the use of tetrazol compounds. It is interesting to note that there as yet is no commercialization of the Poole et al. invention which raises the question as to whether such high aspiration ratios are in fact achievable with any of the prior art designs. Analysis has shown that this is the case, that is, that such large aspiration ratios are not achievable with the prior art designs.
U.S. Pat. No. 4,928,991 to Thorn describes an aspirating inflator assembly including aspiration valves which are generally needed in all high pumping ratio aspiration systems. Sodium azide is the propellant used. Pumping ratios of 1:1 to 1.5:1 are mentioned in this patent which by analysis is possible. It is noteworthy that the preamble of this patent discloses that the state of the art of aspirating inflators yields pumping ratios of 0.1:1 to 0.5:1, far below those specified in several of the above referenced earlier patents. Once again, little attempt has been made to optimize the nozzle design.
U.S. Pat. No. 5,004,586 to Hayashi et al. describes a sodium azide driver side inflator in which the aspirating air flows through a series of annular slots on the circumference of the circular inflator in contrast to the earlier Hayashi et al. patent where the flow was on the axis. Similar pumping ratios of about 4:1 are claimed however, which by analysis is unlikely. Once again, aspiration valves are not shown and the reason that they can be neglected is not discussed. An inefficient nozzle design is again illustrated. The lack of commercial success of these two Hayashi patents is probably due to the fact that such high pumping ratios as claimed are not in fact achievable in the geometries illustrated.
U.S. Pat. No. 5,060,973 to Giovanetti describes the first liquid propellant airbag gas generator wherein the propellant burns clean and does not require filters to trap solid particles. Thus, it is one preferred propellant for use in the instant invention. This system however produces a gas which is too hot for use directly to inflate an airbag. The gas also contains substantial quantities of steam as well as carbon dioxide. The steam can cause burns to occupants and carbon dioxide in significant quantities is toxic. The gas generator is also circular. Aspirating systems are therefore required when using the liquid propellant disclosed in this patent, or alternately, the gas generated must be exhausted outside of the vehicle.
U.S. Pat. No. 5,129,674 to Levosinski describes a converging-diverging nozzle design which provides for more efficient aspiration than some of the above discussed patents. Nevertheless, the airbag system disclosed is quite large and limited in length such that the flow passageways are quite large which requires a long nozzle design for efficient operation. Since there is insufficient space for a long nozzle, it can be estimated that this system has a pumping ratio less than 1:1 and probably about 0.2:1. Once again a sodium azide based propellant is used.
U.S. Pat. No. 5,207,450 to Pack, Jr. et al. describes an aspirated air cushion restraint system in which no attempt was made to optimize the nozzle design for this sodium azide driver side airbag. Also, aspiration valves are used although it is suggested that the exhaust from the airbag can be made through the aspirating holes thereby eliminating the need for the flapper valves. No analysis, however, is provided to prove that the area of the aspiration holes is comparable to the area of the exhaust holes normally provided in the airbag. Although no mention is made of the pumping ratio of this design, the device as illustrated appears to be approximately the same size as a conventional driver side inflator. This, coupled with an analysis of the geometry, indicates a pumping ratio of less than 1:1 and probably less than 0.2:1. The statement that the aspiration valves are not needed also indicates that the aspiration ratio must be small. Large inlet ports which are needed for large aspiration ratios are generally much larger than the typical airbag exhaust ports.
U.S. Pat. No. 5,286,054 to Cuevas describes an aspirating/venting motor vehicle passenger airbag module in which the principal of operation is similar to the '264 patent discussed above. Once again the aspiration pumping ratio of this device is 0.15:1 to 0.2:1 which is in line with conventional aspirated inflators. It is interesting to note that this pioneer in the field does not avail himself of designs purporting to yield higher pumping ratios. Again the nozzle design has not been optimized.
Other U.S. patents which are relevant to the instant invention but which will not be discussed in detail are: U.S. Pat. No. 3,158,314 to Young et al., U.S. Pat. No. 3,370,784 to Day, U.S. Pat. No. 5,085,465 to Hieahim, U.S. Pat. No. 5,100,172 to Van Voorhies et al., U.S. Pat. No. 5,193,847 to Nakayama, U.S. Pat. No. 5,332,259 to Conlee et al. and U.S. Pat. No. 5,423,571 to Hawthorn.
None of the prior art inflators contain the advantages of the combination of (i) a linear inflator having a small cross section thereby permitting an efficient nozzle design wherein the length of the nozzle is much greater than the aspiration port opening, (ii) a non sodium azide propellant which may produce toxic gas if not diluted with substantial quantities of ambient air, and (iii) an inflator where minimal or no filtering or heat absorption is required.
It is interesting to note that in spite of the large aspiration pumping ratios mentioned and even claimed in the prior art references mentioned above. and to the very significant advantages which would result if such ratios could be achieved, none has been successfully adapted to an automobile airbag system. One reason is that pumping ratios which are achievable in a steady state laboratory environment are more difficult to achieve in the transient conditions of an actual airbag deployment.
None of these prior art designs have resulted in a thin linear module which permits the space necessary for an efficient nozzle design as disclosed herein. In spite of the many advantages claimed in the prior art patents, none have resulted in a module which can be mounted within the vehicle headliner trim, for example, or can be made to conform to a curved surface. In fact, the rigid shape of conventional airbag modules has forced the vehicle interior designers to compromise their designs since the surface of such modules must be a substantially flat plane.
With respect to airbag systems including a plurality of inflatable airbags or unconventionally large airbags and inflators therefor, automobile manufacturers are now installing more than two airbags into a vehicle. The placement of both side and rear seat airbags have in fact taken place by at least one manufacturer each; Nissan for rear airbags and Volvo, General Motors and Ford for side airbags. However, Nissan has stated that it cannot provide more than a total of two airbags in the vehicle and that it will not offer a front passenger side airbag for those vehicles that have a rear seat airbag. With respect to the Volvo, General Motors and Ford airbags, these side airbags will not deploy when the frontal airbags do because if more than two airbags would be deployed in a vehicle at the same time, the pressure generated by the deploying airbags within the passenger compartment of the vehicle creates large forces on the doors. These forces may be sufficient to force the doors open and consequently, if the doors of the vehicle are forced open during a crash, vehicle occupants might be ejected, greatly increasing the likelihood of serious injury. In addition, the pressure generated within the passenger compartment creates excessive noise which can injure human beings.
In addition to airbags for side impacts and rear seats, it is likely that airbags will be used as knee bolsters since automobile manufacturers are having serious problems protecting knees from injury in crashes while providing the comfort space desired by their customers.
As soon as three or more airbags are deployed in an accident, provisions should be made to open a hole in the vehicle to permit the pressure generated by the deploying airbags to escape. What has not heretofore been appreciated, however, is that once there is a significant opening from the vehicle to the outside, the requirements for the composition of the inflator gases used to inflate the airbags change and inflators which generate a significant amount of toxic gas become feasible, as will be discussed below.
The primary gas generating propellant used in airbag systems today is sodium azide. This is partially due to the fact that when sodium azide burns, in the presence of an oxidizer, it produces large amounts of Nitrogen gas. It also produces sodium oxide which must be retained in the inflator since sodium oxide, when mixed with moisture, becomes lye and is very toxic to humans. Thus, current inflators emit only nitrogen gas into the passenger compartment which occupants can breath for a long period of time in a closed passenger compartment without danger. The sodium azide inflators also are large to accommodate the fact that about 60% of the gas generating material remains in the inflator with only about 40% emerging as gas.
Other propellants, including nitrocellulose, nitroguanidine, and other double base and triple base formulations, as well as a large number of liquid propellants, exist which could be used to inflate airbags, however they usually produce various quantities of gases containing compounds of nitrogen and oxygen plus significant amounts of carbon dioxide. In many cases, the gases produced by these other propellants are only toxic to humans if breathed over an extended period of time. If the toxic gas were removed from the vehicle within a few minutes after the accident then many of these propellants would be usable to inflate airbags.
Much of the energy released when sodium azide burns in the inflator is removed from the gas by the cooling and filtering screens. In some designs, the sodium oxide must be trapped by the filters which requires that the gas be cooled to the point where sodium oxide condenses. In all designs, the gas is cooled so that the temperature of the gas in the airbag will not cause burns to the occupants. In all current designs, a substantial amount of the energy in a propellant is lost through this cooling process which in turn necessitates that the inflator contain more propellant.
Airbag systems have primarily been installed within the instrument panel or steering wheel of automobiles. As a result, although numerous attempts have been made to create aspirated inflator systems, they have only been used on the passenger side and their efficiency has been low. In aspirated inflator systems, part of the gas to inflate the airbag is drawn in from the passenger compartment. However, in a typical passenger side airbag system in use today where aspiration is employed, substantially less that about 30% of the gas which inflates the airbag comes from the passenger compartment. In view of the large size of conventional sodium azide inflators and woven airbags, there is limited room for the airbag system and it is difficult to design aspirating systems which will fit within the remaining available space. One reason is the resistance of the air flow through the instrument panel into the aspirator. Another reason for the low efficiency of aspirated inflator systems is that the aspirated systems used today have inefficient nozzle designs. Theoretical studies of aspiration systems, such as described herein, show that the percentage of gas drawn in from the passenger compartment could be raised to as high as about 75% or even 90%.
One airbag aspiration method is described in U.S. Pat. No. 4,909,549 (Poole et al.) Poole et al. describes a method for inflating an airbag in which a substantially non-toxic primary gas mixture is diluted with outside air by passing the primary gas mixture through at least venturi to aspirate the air. Poole et al. does not suggest that the outside air should come from the passenger compartment and therefore does not provide a solution to the problem of excessive pressure being generated in the passenger compartment upon deployment of multiple airbags, as discussed above.
If alternate, more efficient propellants are used and if the gas produced thereby is exhausted at a much higher temperature, more of the energy would be available to heat the gas which is flowing from the passenger compartment to the airbag thus further increasing the efficiency of the system and reducing the amount of propellant required. Since cooling screens are not necessary and since the efficiency of the propellant is high, the inflator can be made very small providing the extra space needed to design efficient aspirating nozzles.
Since many alternate propellants produce toxic gases, their use becomes practical (i) if the quantity used is substantially reduced, (ii) if means are provided to prevent the gas from entering the occupant compartment, or (iii) if means exist within the vehicle to exhaust the toxic gas from the vehicle shortly after the airbag is deployed.
Finally, today the airbag electronics are housed separate and apart from the airbag module and the energy needed to initiate the inflator is transmitted to the airbag module after the crash sensor has determined that the airbag deployment is required. This has resulted in many failures of the airbag system due to shorted wires and other related causes. This and other problems could be solved if the crash sensor electronics send a coded signal to the airbag module and the electronics associated with the module decoded the signal to initiate the inflator. The diagnostics circuitry can then also be part of or associated with the module along with the backup power supply which now also becomes the primary power supply for the module.
These and other problems of current airbag systems are solved by the invention disclosed herein and described in detail below.